# Add high-pass option to diode ladder filter

**URL:** <https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409>\
**Category:** DIY STUFF\
**Created:** [April 21, 2021, 8:49am UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409 "2021-04-21T08:49:53Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![nschagen](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@nschagen](https://lookmumnocomputer.discourse.group/u/nschagen)\
**Post date:** [April 21, 2021, 8:49am UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/1 "2021-04-21T08:49:53Z")

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Hey synth builders!

I would like to add the following filter to my rack. It sounds amazing \<3

[![](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/3/3f2369e61b90bc13defac36f50c09b6d26076d68.jpeg "Designing a diode ladder filter from scratch") ](https://www.youtube.com/watch?v=jvNNgUl3al0)

However, I would really like to add a high-pass switch to it. I found that this is possible for the MS-20 filter design ([see this for example](https://www.eddybergman.com/2019/12/synthesizer-build-part-12-korg-ms20.html)). However, would it be also possible for this design in some way?

Thanks in advance!

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**Author:** ![jaradical](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/jaradical/32/12069_2.png) [@jaradical](https://lookmumnocomputer.discourse.group/u/jaradical)\
**Post date:** [April 21, 2021, 9:55am UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/2 "2021-04-21T09:55:31Z")

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G’day there,

Adding a HP mode to a diode ladder filter is something I’m looking into as well.

My understanding is that you can’t do it by simply changing where you connect the input, like you would with an op-amp based filter such as the MS-20 filter you’ve pointed to.

One approach, which I haven’t had much luck simulating, but am trying to breadboard, is to take the LP output of the filter, and subtract it from the original input signal and see if that gets you a HP filter. Care needs to be taken with the phase and polarity of the signals - I’m not sure what it looks like in @moritzklein’s version, but in the EMS/TB-303 style diode ladder filters the LP out is around 180Deg out of phase. Plus, I’m not sure what will happen when you have resonance cranked up…

Sorry, not much concrete help, if I do get anything working I’ll be sure to share. Do let me know how you get on!

Cheers

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**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\
**Post date:** [April 21, 2021, 10:24am UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/3 "2021-04-21T10:24:07Z")

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No HP but a Band pass add , see in this thread 🙂

> [@Designing a diode ladder filter from scratch](https://lookmumnocomputer.discourse.group/t/designing-a-diode-ladder-filter-from-scratch/3118):
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> hi people, here’s the latest episode in my filter design series. this time, we’re building a three pole diode ladder filter (though it could be debated if that is indeed the right name … ah semantics). it has nice smooth resonance and is very stable (unlike some other diode-based implementations)! let me know if you have any questions & thanks for watching! [[Designing a diode ladder filter from scratch] ](https://www.youtube.com/watch?v=jvNNgUl3al0)

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**Author:** ![nschagen](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@nschagen](https://lookmumnocomputer.discourse.group/u/nschagen)\
**Post date:** [April 21, 2021, 12:39pm UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/4 "2021-04-21T12:39:02Z")

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ooohh \<3 love it. I also didn’t have bandpass yet. In that case, I will try and add a low-pass / band-pass switch to the design instead 🙂

> One approach, which I haven’t had much luck simulating, but am trying to breadboard, is to take the LP output of the filter, and subtract it from the original input signal and see if that gets you a HP filter.

Interesting approach indeed. Might not be easy since you have to get the phase/amplitude completely right, so in all cases, it requires extra fiddling in practice. This is a demonstration of the technique:

[![](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/9/e/9eb1f1732cddb0c9cab5ae48e3ea7639d8e72072.jpeg "Hipass from Lopass filter") ](https://www.youtube.com/watch?v=SKcGhaduvfs&t=137s)

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**Author:** ![moritzklein](https://avatars.discourse-cdn.com/v4/letter/m/d78d45/32.png) [@moritzklein](https://lookmumnocomputer.discourse.group/u/moritzklein)\
**Post date:** [April 22, 2021, 1:03pm UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/5 "2021-04-22T13:03:26Z")

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next video will be exactly on this topic, should be up in about two weeks!

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**Author:** ![nschagen](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@nschagen](https://lookmumnocomputer.discourse.group/u/nschagen)\
**Post date:** [April 22, 2021, 1:22pm UTC](https://lookmumnocomputer.discourse.group/t/add-high-pass-option-to-diode-ladder-filter/3409/6 "2021-04-22T13:22:25Z")

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Thanks for all the videos mate! I love them! Can’t wait to build it!